So you know how toggling autopilot has a timeout, but self-destruct doesn't? Well, this can work for you! If you always keep self-destruct counting down, it'll take less than five seconds to destruct if you need to.
To keep it on without accidentally blowing yourself up, start up a drumbeat synthesizer program. http://www.drumbot.com/projects/sequence/ is the first one I tried, and it works well enough.
Put it into 5/4ths time
at 60bpm,
and add a low beat every second, so it counts out the five seconds to explosion. Set it to play on loop.
Add a high beat on the first second so that it plays along with the first low beat.
Now for the saving note: Add a high beat quarter-note at the very end of the fifth second (move it backward if you're not good at timing, or make it smaller if you really want to shave some time)
If you count along, it goes
FIVE ee and a four ee and a three ee and a two ee and a one ee and A, or
FIVE... 4... 3... 2... 1... AND FIVE... 4... 3... 2... 1... AND FIVE...
where AND FIVE are two high beats played one after the other.

When you spawn, wait for the first high beat, then start the self-destruct on the second high beat. Press it on both the next two high beats. The first of the two will stop the timer a quarter second before you would have died, the second starts the timer so that the countdown is synchronized with the beats.

I recommend using an external keyboard placed on the floor operated by a foot as a floor pedal, using numpad enter as the self-destruct button, so that it doesn't mess up the rhythm of your 'other' hands for moving and shooting.

Allowing the self-destruct timer reset to lapse lets you save your team from being genocided (if team score is important to you) should you find yourself in the air falling in front of someone holding such a flag.
It's also useful against a regular tank (if personal score is important to you), where a regular kill would offset your scores by two (they gain a point, you lose a point), whereas a self-destruct instead of a guaranteed kill only offsets your relative scores by one (you lose a point, they stay the same).

That said, it is easier to only count out the seconds than to actually remember to stop going along with the music when you know you're going to die. You also might not have enough time, or think a height drop will take longer than it actually does, or even focus more on the timing and less on the gameplay.

I put this here to see your responses and see if anyone can create a usable strategy out of this.